2/3 times pressing a button does nothing.

I am using the Input.GetButtonDown function to detect input for a lot of my buttons. I want the inputs to only be able to cause 1 action which is why I'm using GetButtonDown. The problem is that it doesn't always register, even if I press the button all the way down. This will happen more frequently of course if I press the button faster, but sometimes even if I haven't pressed the button recently, it still does not register.

I am calling these button presses from FixedUpdate. Is there a way to register every time the button is pressed to keep from missing any GetButtonDowns?